PURPOSE: To efficiently remove an oil under a condition suited to a fertilizer or feed without necessity of expensive complex large-sized equipments, by inserting oil-utilizing or decomposing microorganisms to scum cake formed from waste processing water used for mass-fishing fishery products, and fermenting the scum cake.

CONSTITUTION: To scum cake having a water-contng. ratio of about 60W70%, discharged from waste water in processing marine products such as sardines, mackerels, cuttlefihes or the like, is inserted one kind of microorganism or the mixture of microorganisms, and the scum cake is fermented. The microorganism may be Psendomonas sp or Bacillus sp as bacteria, Aspergillus sp as mold or Candida or the like as yeast. By this treatment, an oil part in the scum cake is reduced, and an excellent oil-removing effect is obtained.